test: migrate from AVA to Node.js native test runner (#346)

AVA stores snapshots in a binary format (`.snap`), which produces no
meaningful diffs and bloats Git history. This replaces AVA with the
built-in `node:test` module, whose snapshot support generates
human-readable text files that are easy to diff and review in pull
requests.

The migration also replaces `@sinonjs/fake-timers` and `execa` with
Node.js built-ins (`node:test` mock timers and `node:child_process`),
removing three dev dependencies total.

### Changes

- **`tests/index.js`**: Rewritten to use `node:test` with a custom
snapshot serializer that renders strings with actual newlines. Uses
subtests for labeled `stderr`/`stdout` snapshots, and only snapshots
non-empty output.
- **`tests/main-repo-skew.test.js`**: Replace `@sinonjs/fake-timers`
with `mock.timers.enable()` from `node:test`.
- **`tests/README.md`**: Updated documentation to reflect `node --test`
and the new snapshot file.
- **`package.json`**: Remove `ava`, `@sinonjs/fake-timers`, and `execa`
from devDependencies. Update test script to `c8 --100 node --test
tests/index.js`.
- **`tests/index.js.snapshot`**: New text-based snapshot file replacing
binary `tests/snapshots/index.js.snap`.
- **`tests/snapshots/`**: Deleted.

All 22 test scenarios (66 subtests) pass with 100% code coverage.

Closes #344

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

tests/index.js

@@ -1,15 +1,23 @@
 import { readdirSync } from "node:fs";
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
 
-import test from "ava";
-import { execa } from "execa";
+import { snapshot, test } from "node:test";
+
+const execFileAsync = promisify(execFile);
+
+// Serialize strings as-is so multiline output is human-readable in snapshots
+snapshot.setDefaultSnapshotSerializers([
+  (value) => (typeof value === "string" ? value : undefined),
+]);
 
 // Get all files in tests directory
 const files = readdirSync("tests");
 
 // Files to ignore
-const ignore = ["index.js", "main.js", "README.md", "snapshots"];
+const ignore = ["index.js", "index.js.snapshot", "main.js", "README.md"];
 
-const testFiles = files.filter((file) => !ignore.includes(file));
+const testFiles = files.filter((file) => !ignore.includes(file)).sort();
 
 // Throw an error if there is a file that does not end with test.js in the tests directory
 for (const file of testFiles) {
@@ -18,20 +26,31 @@ for (const file of testFiles) {
   }
   test(file, async (t) => {
     // Override Actions environment variables that change `core`’s behavior
-    const env = {
-      GITHUB_OUTPUT: undefined,
-      GITHUB_STATE: undefined,
-      HTTP_PROXY: undefined,
-      HTTPS_PROXY: undefined,
-      http_proxy: undefined,
-      https_proxy: undefined,
-      NO_PROXY: undefined,
-      no_proxy: undefined,
-      NODE_OPTIONS: undefined,
-      NODE_USE_ENV_PROXY: undefined,
-    };
-    const { stderr, stdout } = await execa("node", [`tests/${file}`], { env });
-    t.snapshot(stderr, "stderr");
-    t.snapshot(stdout, "stdout");
+    const {
+      GITHUB_OUTPUT,
+      GITHUB_STATE,
+      HTTP_PROXY,
+      HTTPS_PROXY,
+      http_proxy,
+      https_proxy,
+      NO_PROXY,
+      no_proxy,
+      NODE_OPTIONS,
+      NODE_USE_ENV_PROXY,
+      ...env
+    } = process.env;
+    const { stderr, stdout } = await execFileAsync("node", [`tests/${file}`], {
+      env,
+    });
+    const trimmedStderr = stderr.replace(/\r?\n$/, "");
+    const trimmedStdout = stdout.replace(/\r?\n$/, "");
+    await t.test("stderr", (t) => {
+      if (trimmedStderr) t.assert.snapshot(trimmedStderr);
+      else t.assert.strictEqual(trimmedStderr, "");
+    });
+    await t.test("stdout", (t) => {
+      if (trimmedStdout) t.assert.snapshot(trimmedStdout);
+      else t.assert.strictEqual(trimmedStdout, "");
+    });
   });
 }